Light emitting diodes (LEDs) are emerging as the superior technology for lighting applications, offering energy efficient, eco friendly, robust and long-lasting alternative to traditional incandescent and fluorescent lamps. In view of global attention to energy saving the long term goal is to replace the conventional light sources with LEDs. This requires further major improvements such as cost reduction and enhanced light quality.
A common approach to white light generation is the downconversion of high energy light, for example, from the UV, violet or blue end of the electromagnetic spectrum, into broadband visible light. Such an approach is used in fluorescent light sources, in both tube and compact fluorescent devices, as well as many white LEDs. The downconverting materials that are commonly used for this purpose includes mixtures of rare earth materials. Typically, these rare earth materials have high efficiencies in converting UV or violet light into visible light, and are thermally and chemically stable. Thus, they may withstand the harsh environments inside fluorescent sources. While these rare earth materials work well as downconverters for white light sources, they are expensive and in limited supply. Therefore, there exists a great deal of interest in seeking solutions for energy downconversion for lighting that does not involve rare earth metals.
Some of the most efficient white light emitting inorganic LEDs (iLEDs) use a downconverting strategy, similar to fluorescent tube sources to produce their broadband emission. These sources typically rely on rare earth metals for converting high energy light into a broadband (white) spectrum of emission. While it is possible to generate white light with several different LEDs running in parallel, e.g., separate red, green and blue iLEDs, this adds cost to the system and suffers from the “green gap” problem, which is associated with the observation that green LEDs show a marked drop in efficiency when they are run at high brightness. The green portion of the light produced by multiple iLEDs is produced inefficiently, thus limiting the overall system efficiency. Such drop in efficiency is smaller for UV, violet and blue emitting devices. Therefore, devices that are able to downconvert UV/violet/blue light into the visible spectrum can be run at high brightness and high efficiency. Thus, there is a need to design and employ downcoverting materials that can be efficiently excited by light from UV, violet or blue iLEDs, to generate a broad band spectrum.